How to define the discharge pressure setpoint

[Lobijirizillo]

Dears,
I have the following pump details:

Capacity, Normal: 170 m3/h; Rated: 187 m3/h.
Suction: 1.8 barA.
Discharge: 10.1 barA.
Dif: 8.3 barA.
Service: Flash drum pump, produced water.

I would like to know how i can set up the suction/discharge trip setpoint.
Which criteria do i have to consider?
Best Regards,
ML.

 

[BigInch]

You need to know the system curve, how much resistance the downstream piping and equipment give for each flowrate in the operating range, then the sensitivity of your required flowrate to changes in pressure. Hopefully the downstream items have a higher pressure rating than your 10.1 bara, otherwise you should also have a relief valve set for whatever their design pressure is.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[Lobijirizillo]

BigInch, thanks for your reply.
I have the pump system curve, it might help us. Nowdays, our setpoint is 5 bar. is it correct?; if we change it to 4 bar, is still correct? is there any relationship/percentage between discharge setpoint and discharge pressure?

System works with produced water, S.g: 1.12

Thanks for your help and support;
ML.

 

[BigInch]

No. That's only half of what you need. You must combine the pump curve with the system curve. The system curve describes the head required vs flow required points only of your downstream pipe and equipment. You superimpose the pump curve onto the system curve to get the pump & system curve diagram. Once you know the pressure you need for your operating flowrate, you can begin to make some ideas about pressure set points. You must also know the lowest design pressure rating of all the components downstream.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[Pumpsonly]

What are you trying to protect? .. The pump, the motor or the piping ?

 

[Lobijirizillo]

Hi Pumpsonly,

we are trying to protect the pump.

 

[JJPellin]

If your only need is to protect the pump, then using suction and discharge pressure trips is unusual. I would be more likely to set a trip on low suction level in the drum to keep from running the pump dry. I would be more likely to set a trip on low discharge flow to keep the pump from running dead-headed or below Minimum Continuous Stable Flow. If you don't have instruments for level and flow, you could use pressure with some loss of protection. A low suction pressure could be substituted for low suction level. A high discharge pressure could be substituted for low flow.

To determine the proper set-points for these substitute trip set-points, you would need to know the full range of possible suction conditions and the pump curve. If the pressure in the flash drum is constant and stable, then suction pressure at the pump would represent drum pressure plus head pressure. If the drum pressure did not change, then any change would represent a change in level. The discharge pressure of the pump could be correlated to flow by assuming a given suction pressure and using the pump curve.

Depending on the variability of your suction pressure, this could be a very poor way to protect the pump. You could substitute a low amp trip based on field testing at low flow. A low amp trip might also provide protection for loss of suction level. Depending on the characteristics of the pump, this might not provide good protection, either.


Johnny Pellin

 

[Pumpsonly]

If yours is a centrifugal pump, it is unlikely you need to protect it from over pressure.
If you need to protect the pump from dry running or cavitation due to low suction level,a combination of suction pressure and level monitoring will be more appropriate. You will need to take into consideration the vapour pressure of the liquid in the determining the set point and also the NPSHR curve of the pump.Baiscally you have to work out the worse suction condition that will cause the pump to cavitate.